The carbon electrode holders for an arc lamp used as a light source in a conventional weatherometer or lightfastness tester mostly comprise receptacles for receiving three or four carbon rods which act as the electrodes, and metal support members for pressing the carbon rods into the receptacles, as illustrated in FIGS. 1-3. Typical configurations of these conventional holders are also illustrated in FIGS. 2 and 3.
The receptacle 1 of the holder shown in FIG. 2 has four holes 2 thereon, each being defined by an upper ring a and a lower ring b. A carbon electrode C is inserted into each hole 2 and extends through the upper and lower rings a and b, and the electrodes are clamped and fixed in position by T-shaped metal supports 3 and 3' which are attached to the receptacle 1 by a bolt 4 extending transversely through the receptacle 1 relative to the rods and having nut 5 on the end thereof.
The receiving section of the receptacle 6 of the holder shown in FIG. 3 has rectangular recesses therein in which two carbon electrodes C are held, each being pressed against two perpendicular surfaces c and d. The metal supports 7 and 7' have the shape of a wedge, and are clamped by the bolt 8 extending transversely through the receptacle relative to the carbon electrodes C and having a nut 9 thereon. Since the support member has a wedge shape, the carbon electrodes C are each pressed against the two surfaces c and d and held thereagainst by a strong force.
The carbon electrodes C in these conventional holders shown in FIGS. 2 and 3 are held by means of the clamping action of the bolt and nut. If the bolt and nut somehow come loose, therefore, the carbon electrodes also get loose, and sometimes slip out of the receptacle during use of the apparatus.
In addition, the electrodes in the conventional holders like those of FIG. 3 are in linear contact in the longitudinal direction thereof with the receptacle. For this reason, the effective contact area of the carbon electrodes is rather small. Since a large current, such as for example 60A, passes through this small contact area, thermal oxidation tends to take place at this contact point, if the electric contact-resistance increases even to a small extent. The thermal oxidation, in turn, deterioriates the electric contact, thus resulting in an endles vicious cycle.
In order to prevent these undesirable sequences of events, it has heretofore been imperative to polish the contact surface to make it clean and thus ensure a good electric contact every time the apparatus is used.